Eye Examinations
Eye health-care practitioners generally divide patient examination into three parts: examination of the cornea, examination of the retina, and a vision function exam including measurement of the refractive status of the eye. The doctor's findings need to be recorded and the standard method for last century has been to make hand-written notes in the chart. Hand recording of vision function and refractive status is completely satisfactory. Vision function is basically a quantitative assessment by the doctor and six numbers describe the refractive information of both eyes so that the manual recording process is quick and efficient. Recording the clinical status of the cornea and retina is completely different.
For the retinal and corneal eye-health exams what is needed is quantitative clinical data but what has usually been recorded in the past is the doctor's clinical assessment. For example, an examiner may record, “the optical disk has a normal pallor” which is the clinical perception or, even more simply, the diagnoses, “this patient does not have glaucoma”. Seldom is the actual clinical information recorded, which, in this instance, would be a color image of the optical disk. This lack of documentation leaves open an opportunity for later criticism that the examination or diagnoses was faulty. Further, it is well known in the instance of estimating the pallor, the cup-to-disk ratio, and the like, that making assessments of these quantities are difficult and that the intra-observer variation is large. Especially for these examples, it would be quite beneficial to have a method for making a detailed comparison of changes in the optical disk between exams.
Most retinal exams are accomplished by using the optical aids of the direct ophthalmoscope, binocular indirect ophthalmoscope (BIO) or a special lens with the slit-lamp/biomicroscope.
Direct Ophthalmoscope
The direct ophthalmoscope consists of a light and single lens held between the doctor's and patient's eye by which the doctor can visualize a very small segment of the retina at a time. The light is considered uncomfortably bright by most patients and skill is required on the part of the clinician. By scanning the visualized area about, a mental image of the posterior pole may be obtained for a basic assessment of retinal health. It is difficult to simply stop the scan and study a given area such as the optical disk because of patient motion and discomfort.
Binocular Indirect Ophthalmoscope
For a more complete visualization of the retina, a BIO may be used. The BIO comprises a lens mounted on a headband in front of each of the doctor's eyes, a single lens held by hand close to the patient's eye, and a light also mounted on the doctor's headband. The field-of-view visualized is wider than that of the direct ophthalmoscope and this instrument is generally used through dilated pupils. With the BIO the doctor can more thoroughly examine the periphery of the retina. Using the BIO requires a great deal of clinical skill and is usually learned over a period of an entire year while the doctor is in training. However, like the direct ophthalmoscope, the doctor must develop a mental picture of the broader features of the eye and, because of the bright light and movements of the patient's eye, it is difficult to stop and carefully study one portion of the retina.
Slit-Lamp Biomicroscope
The slit-lamp is designed for corneal visualization. This instrument is a binocular microscope and a small lamp that projects a narrow rectangle of light into the anterior structures. This microscope, with a special lens and the slit-lamp light, can be used for retinal visualization as well. However, when modified for retinal imaging, its inherent limitations generally prevent it from providing high quality retinal visualizations. The examination can only be done on patients with a dilated iris. The lens is positioned to be very close to the patient's eye, which in turn makes it very difficult to determine and adjust the alignment for the lens. The contact type lens can be very uncomfortable to the patients. The lens produces strong light reflection from its surfaces, which deteriorate the quality of retinal image greatly. However, with only a slit of light, only small portions of the retina can be observed at a time and patients generally feel that the light intensity if very uncomfortable. Overall, modifications on the slit lamp biomicroscope produce a very substandard retinal visualization system.
Dilation and Bright Lights
Currently, for a through eye exam, and almost always when the BIO is used, it is necessary to dilate the patient's eye. Dilation comprises the application of eye drops that open the iris to a larger than normal diameter and can not be applied until the refraction portion of the exam is completed. Significant time is required for the drops to take effect. During this time the patient is almost always taking up limited space in the examination room. Further, dilation is very objectionable to patients because of the elapsed time for the dilation to return to normal. Studies show that this alone is a major factor for patients to defer having eye exams. Most patients also find the brightness of the light objectionable and many times to the point of pain. While some BIO's come equipped with head-mounted cameras, these have not been widely accepted, are regarded as difficult to use, and only image a small portion of the retina at a time in any instance. A hazard of dilation is the risk of inducing acute glaucoma that can lead to immediate blindness. Thus, a system that can accomplish an exam with little or no dilating eye drops and no bright light would be of great advantage.
Prior Art Eye Cameras
Fundus Cameras
For accurate documentation sometimes fundus cameras are used as a supplement or replacement for the manual retinal exam. These cameras have been in use since the 1940's and most of them record images of the retina on film. Film has the disadvantage of requiring processing before an assessment of image quality can be obtained and there is no ability to immediately electronically transfer the image. Some cameras are now being equipped with digital imaging add-on capability. By digital imaging we mean the use of an electronic image sensor such as CCD or CMOS followed by digitization and digital storage means.
In current practice these digital add-ons to existing cameras and are quite bulky and expensive. As a consequence, fundus cameras, digital or film, are usually located in a separate room and a specialized technician is employed to operate them. The high level of acquisition and operating costs for digital cameras has left digital imaging to the domain of high-end clinical sites and they are not used for routine exams. Digital cameras have also been added to slit-lamp biomicroscopes so that they can be used for imaging, but this single purpose application has generally proved to not be cost effective and is seldom implemented.
Use of Fundus Cameras for Cornea Viewing
Although designed for retina imaging, the fundus camera has been used to image the cornea. However, the camera generally produces low quality pictures because the inherent achromatic and spherical optical aberrations when used with an air path and are high and the camera has only a very limit working range. When the cornea is in focus, the patient's eye is located so close to the camera that it becomes difficult to place a slit-lamp between them and no known commercial product provides a slit-lamp with the fundus camera. If a slit-lamp were added, the lamp would block or distort the view of camera when it is positioned in the front of the objective lens. The built-in internal magnification adjustment for the fundus camera is not adequately designed for the required magnifications of corneal imaging. Thus, as a practical matter, using the fundus camera for corneal imaging is very non-optimal.
Scanning Laser Ophthalmoscope
In yet another prior art retinal imaging approach, a mechanically driven mirror is used to scan a laser beam about the retina and the reflected intensity is measured to generate an image. These imaging systems, commonly called a scanning laser ophthalmoscope or SLO, usually only provide one laser wavelength and this therefore does not produce a color image, a significant clinical disadvantage. Recently, a system was provided to the market with two laser colors, but even this produces very, very poor color image quality. Even greater limitations are in the relatively long exposure time that allows eye movement during the frame time, the large size, and the high cost.
Prior Art Laser Eye Surgery
The laser has been widely used in treatment of various diseases in the anterior and posterior segment of the eye. The BIO or biomicroscope is one method used to deliver the laser to retinal or corneal region. To align the clinician's eye, the condensing lens and patient's eye must be in line for viewing, and at same time the laser spot must be directed to the intended area. This is a very challenging task. The slit-lamp biomicroscope, with additional laser delivery attachment and a laser lens (contact or noncontact), is the most commonly used platform. Although it provides a more stable condition for laser procedure, the external attachment makes the system complicated to use. The laser lens is very often being held by one hand of the clinician. Any motion of the lens causes the viewed retinal image to move, especially in the case of high magnification lens. It is not comfortable to hold the laser lens steady during the long laser treatment session which can last for several minutes. The regular illumination to the retina is provided by the slit-lamp lamp in this case. To avoid blocking the laser beam the clinician must maintain certain positions with the slit-lamp while simultaneously projecting the light to the desired area. In addition, the reflected laser light from the laser lens can scatter back in many directions in the room, a result hazardous to others present. Viewing through the biomicroscope and laser lens, the clinician can not simultaneously see the iris and make the judgment on the state of alignment for the lens. As result, there is the risk of accidentally firing the laser on to the iris. The nature of the manual manipulation of the laser beam also makes it difficult to assess the dosage of laser being delivered to the retina if no clear marks are left after the treatment. In a new treatment, photodynamic therapy, the laser power level is below that which would leave a mark on the retina. This, the control of the laser dosage is very critical in PDT treatment. Sometimes, a completely separate system is provided for laser treatment, adding to the expense to well equip an eye doctors office.
What is needed is a relatively low cost, digital, and eye camera for monitoring and recording the conditions of the retinal and corneal regions of the eye. This system would have even greater value if it could be additionally used for laser treatment and retinal stimulus for visual function testing.